CD154 (CD40 ligan) expression by activated of CD 154-CD40 interactions has been shown to have salutary effects in models of autoimmunity, inflammation, and graft rejection. Despite this, little is known about the regulation of its expression, except that it appears to have several features that distinguish it from cytokine genes. Several studies have indicated the importance of mRNA stability as a major determinant in regulating CD154 expression. Work in our laboratory has delineated a novel region of the human CD 154 >UTR that regulates chimeric reporter gene expression. This cis-acting element is active in both murine and human cells, is highly conserved, and mediates effects equivalent to that of AU-rich elements (AURE) which have been shown to be central in the regulation of cytokines such as TNF-a and IL-2. Careful analysis of mRNA stability in normal T lymphocytes is limited by technical considerations. Therefore, we propose to test the role of this cis-acting element in CD 154 gene expression by mutating it by homologous recombination in the murine germ line to generate a mouse line in which this element is no longer functional. Through this approach, we will determine the role of post-transcriptional regulation of CD 154 expression in health and disease, as was compellingly demonstrated with the TNF-alpha AURE mutant mice. As a result, we will create a model system with which to study its unique regulation and possibly develop novel approaches to regulating its biosynthesis.